In semiconductor laser diodes, including monolithic integrated diode laser arrays, such as laser bars, temperature changes at the active diode junction can result in significant changes in optical output power. Such temperature changes are typically caused by self-heating of a lasing element or crosstalk heating between adjacent lasing elements in an array. Laser diodes have output power sensitivities that typically range from 0.3%/.degree.C. to 0.7%/.degree.C. for constant current operation. This means that a mere 10.degree. C. change in junction temperature can result in a 3% to 7% change in optical output power. Diode laser packages are usually provided, therefore, with thermoelectric coolers that act as heat pumps in order to approximately maintain a constant junction temperature.
Semiconductor laser diodes, especially laser bars, are increasingly used in laser printers, film recorders and other marking or imaging applications. This requires that the laser diodes be modulated in accord with the image to be written. The light emitting elements in such modulated lasers can have outputs which range in power from tens of milliwatts to several watts. Alternatively, digital modulation to fixed power levels may be used, where the digital signals are arbitrary and effectively random. In some instances, laser bars combining ten to twenty separately addressable elements in an array are used with peak modulated outputs on the order of one or more watts per element. At these power levels, the laser diode junction temperature rise relative to the ambient temperature can range from 5 or 10.degree. C. to as much as 20.degree. C. with most thermal management schemes of practical interest. In the case of imaging or marking applications, the essentially random, image dependent modulation of the laser elements creates thermal uncertainties of 10.degree. C. or more and thereby results in unacceptable uncertainties of 3 to 7% or more in optical output from these elements. This uncertainty will produce imaging errors when the laser output is calibrated only to drive current.
It is possible, in principle, to use computer software to compensate for thermal variations based on a laser element's recent modulation history, adjusting the laser element's drive current accordingly. However, this approach is often not practical since modulation rates may extend into the megahertz ranges, and in an imaging application, the modulation history may need to be tracked over time periods of several seconds. Thus, this electrical compensation technique would be very complex. Additionally, in the case of separately addressable arrays, the software would need to account for cross-element heat spreading or `thermal crosstalk` by factoring in heat contributions due to the independent modulation histories of adjacent laser elements of the array.
Laser bars with separately addressable lasing elements present yet another problem related to heating, namely misalignments between the multiple lasing elements of the laser bar and a microlens array used to image the array of emitted light beam, which are caused primarily by thermal expansion of the bar. Preferably, the lens registration variance should not exceed a few tenths of a micrometer, despite the non-uniform heating of the laser bar caused by the vastly different drive signals applied independently to each of the separate lasing elements in the bar. However, while a single thermoelectric cooler will lower the average junction temperature for the laser bar by simultaneously cooling all of the lasing elements in the array collectively, it will not reduce the temperature differential between `on` and `off` elements across the array. In fact, because the single thermoelectric cooler in acting as a heat pump presents a very high incremental thermal impedance to the individual laser elements (high cooling is not the same as low thermal impedance), the cooler will actually increase the temperature differential, while lowering the average temperature by lowering the temperature of the `off` elements by a larger amount than the `on` elements. The result is a source-to-lens registration that exceeds the preferred tolerances. This effect can be counteracted only by allowing cross-element heat spreading prior to reaching the thermoelectric (or other) cooler, in effect causing substantial interelement thermal crosstalk.
Multiple individually controlled thermoelectric coolers might be used, but this runs into some of the same complexity problems as the software compensation techniques referred to above. Further, due to their high thermal impedance, thermoelectric coolers, while able to suppress changes in temperature over relatively long time periods (about 0.1 second or longer), are not able to suppress the self-heating and cross-talk heating that occurs over shorter time periods (about 0.1 second or less). Such short period heating is a major contributor to interelement temperature differential.
Accordingly, it is an object of the invention to provide a temperature control scheme to reduce the temperature variations over time associated with the random nature of the signal driving a laser element's modulation and also to reduce the temperature variations across a laser bar that occur over shorter time periods.